Oh and I seriously doubt that ammo supply would have been an issue for a few dozen rapid fire guns given that the qm corps had few problems supplying upwards 1 million men in the field armed with rifled muskets.

Perhaps so, but line officers always resisted the consideration of rapid firing rifles for general issue, always citing the "waste of ammunition." Generally it would seem that the Army just didn't want one, at least at the expense of long range power. I forget the exact specification, but the rifle had to be capable of "putting down a man and horse" at something like 900 yards. This is partially why the Springfield .45-70 was adopted over the Spencer.